The present invention relates to a radio frequency (RF) probe for a nuclear magnetic resonance apparatus for detecting nuclear magnetic resonance (hereinafter, referred to as an NMR) signals from hydrogen, phosphorus, and the like in a subject and for visualizing a density distribution of a nucleus, a relaxation time distribution, or the like.
A nuclear magnetic resonance imaging (MRI) apparatus is an apparatus such that a high frequency magnetic field is applied to a subject put in a magnetostatic field, NMR signals as a kind of high frequency magnetic field which are consequently generated from the subject are detected by RF probes, and the detection signals are subjected to processes such as Fourier transformation, image reconstruction, and the like, thereby obtaining a visual image. As such RF probes, various kinds of coils for a head region, coils for an abdominal region, local surface coils, or the like which surround a region of interest of the subject (for example, human being) are used.
Among those RF probes, although a sensitivity of the surface coil is higher than that of the coil for the head region or the coil for the abdominal region, a field of view is limited. Therefore, a method whereby a plurality of surface coils are properly slightly overlapped and arranged so that each surface coil doesn't magnetically mutually couple with the adjacent surface coil and NMR signals received by the surface coils are synthesized, thereby substantially widening the field of view, namely, a multiple coil (also called a phased array coil) has been proposed. A principle of the above method has been disclosed in, for example, JP-A-2-500175, JP-A-2-13432, or Magnetic Resonance in Medicine, Vol. 16, pages 192 to 225, (1990). An example in which the phased array coils are applied for an abdominal region has been disclosed in Magnetic Resonance in Medicine Vol. 18, pages 309 to 319, (1991).
FIG. 1 shows a schematic diagram of a conventional multiple coil for an abdominal region. The multiple coil comprises four unit coils 10a to 10d. Namely, the multiple coil is constructed by the upper coils 10a and 10b as a twin coil and the lower coils 10c and 10d as a twin coil. A human body is arranged between the upper coils and the lower coils, while setting the direction of vertebra into a z direction. Each coil is designed so as to resonate at a frequency of a magnetic resonance signal to be detected. The signals from the unit coils are amplified by amplifiers 20 and signal processed, respectively. After that, they are synthesized, thereby forming one image signal. Since the signal from each coil has a high signal-to-noise (S/N) ratio, the image formed by synthesizing also has a high picture quality. However, a distance between the upper coils and the lower coils is fixed, if it is changed, the S/N ratio will remarkably deteriorate.
That is, in the conventional multiple coil, characteristics have been optimized for only an arrangement of the fixed specific coils.
On the other hand, in case of the probes for an abdominal region or the like, in order to allow the probes to be always closely adhered to the shape of the abdominal region in an optimum state, RF probes which can vary the interval between the upper and lower coils so that they can cope with various shapes of human bodies are strongly demanded.
For example, in case of constructing such that the multiple coil of FIG. 1 can be vertically moved as it is, when the coil interval is large, the high frequency electromagnetic coupling between the coils which face each other is relatively weak. However, when the coil interval is small, the high frequency electromagnetic coupling between the coils is strong. Therefore, when the probe is made vertically movable while keeping such a shape, so long as the distance between the coils which face each other is small, a resonance frequency is largely deviated due to an increase in high frequency coupling between the coils. Even if the resonance frequency is adjusted, the S/N ratio remarkably deteriorates. Now, the high frequency coupling mainly comprises the magnetic coupling.
In the conventional multiple coil as mentioned above, when the interval between the coils is made variable, there is a problem such that the magnetic coupling between the coils changes due to a change in interval and the picture quality is deteriorated.